In our previous studies, we have demonstrated that the pathogenic mutation in Huntingtin (Htt) disrupts the normal development of striatal medium spiny neurons (MSNs) in a Huntington's disease (HD) knock-in mouse model. These observations are consistent with other reports suggesting that Htt is required for the attainment of fundamental developmental milestones. Our central hypothesis is that Htt plays pivotal roles in neural stem cell (NSC)-mediated striatal neurogenesis and the functional compartmentalization of the striatum. Furthermore, we postulate that disruption of these Htt-associated neurogenic functions leads to the elaboration of developmentally deregulated MSNs with an enhanced susceptibility to adult stressors, thus contributing with HD-associated profiles of cellular vulnerability. To substantiate this hypothesis, we will employ a floxed Htt mouse model to conditionally ablate Htt in striatal Gsx2+ NSCs and Islet1+ MSN precursors (Gsx2-Cre/Httflox and Islet1-Cre/Httflox, respectively) and define the developmental and late-life consequences of these genetic manipulations. Therefore, the Specific Aims of the current application are to: (1) Determine the developmental roles of Htt in striatal NSCs and Neural Precursor Cells (NPCs) by examining striatal NSC self-renewal, multi-lineage potential and neural lineage specification after conditional ablation of Htt in Gsx2+ species. (2) Determine the developmental roles of Htt in progressive maturation of striatal MSNs by examining the patterns of MSN BrdU birth-dating and the profiles of elaboration of developmental stage-specific MSNs following Htt ablation in either Gsx2+ or Islet1+ neural species. (3) Determine the late-life effects of the selective ablation of Htt prior to and following MSN specification by examining the postnatal consequences of loss of Htt function for the occurrence of HD-associated striatal pathology and motor and behavioral abnormalities. Significance: These studies will define the stage-specific developmental roles of Htt in the program of striatal MSN neurogenesis. Moreover, these studies will provide important insights into the mechanisms mediating the differential profiles of neuronal vulnerability in HD, especially with regard to the role of the loss-of- developmental function mechanism in contributing to HD pathogenesis. Further, these studies have the potential to shift the focus of current research directions by identifying a novel window for the study of HD pathogenesis and for the development of restorative interventions at a time when irrevocable neurodegenerative changes have not yet occurred. The findings obtained from these studies may also have important implications for our understanding of other neurodegenerative diseases, particularly those associated with abnormal repeat expansions.